Put potatoes in a large saucepan with 3 cm cold water and 1 teaspoonsalt; bring to a boil.
Cover, reduce heat and simmer 25 minutes, until potatoes are tender;drain and cool.
Peel potatoes and slice 1 cm thick; set aside.
Heat oil in a medium-size skillet; saute onion until soft.
Stir in flour, mustard, celery seed and remaining one third teaspoon salt.
Gradually add half cup water and vinegar; cook over low heat, stirringconstantly, until mixture boils and thickens.
Combine potatoes, green pepper and carrots in a bowl; add sauce, mixingwell.
Spoon half the mixture into a shallow 8x8-inch baking dish; sprinklewith half cup cheese.
Cover with remaining potato mixture and cheese.
Bake, uncovered, in preheated 350'F. oven 15 to 20 minutes, or untilcheese is melted and vegetables are hot.